The present invention relates to fiber-optic communication systems, and more particularly, to optical amplifiers for use in fiber-optic communications systems.
Optical amplifiers such as erbium-doped fiber amplifiers are used in fiber-optic communications systems to amplify optical signals. Optical amplifiers are sometimes used as power boosters to increase the power of optical signals being launched from a transmitter. Optical amplifiers are also used as line amplifiers to compensate for optical signal attenuation in spans of transmission fiber. Optical amplifiers may also be used as preamplifiers to improve receiver sensitivity.
Some optical amplifiers have adjustable gain to ensure flexibility. Erbium-doped fiber amplifiers, for example, may use variable optical attenuators to adjust their optical gain. As the optical attenuation of a variable optical attenuator is increased, the optical gain of the amplifier decreases.
Variable optical attenuators have previously been used to adjust gain using a linear xe2x80x9cdB to dBxe2x80x9d ratio. With this type of scheme, one adjusts the variable optical attenuator to produce X dBs of loss to decrease the gain of an optical amplifier by X dBs. As a result, the tilt or gain ripple of the optical amplifier does not change when the optical amplifier gain is adjusted. The entire gain spectrum shifts without changing the relative gain at different wavelengths.
It is an object of the present invention to provide arrangements for adjusting the gain of optical amplifiers using sublinear attenuation control.
This and other objects of the invention are accomplished in accordance with the present invention by providing optical amplifiers with variable optical attenuators that are adjusted using a sublinear control scheme. Instead of increasing the attenuation of the optical attenuator by Y dBs for each desired Y dBs in decreased optical amplifier gain, the attenuation of the variable optical attenuator is increased by less than Y dBs. This sublinear control scheme results in a tilt or ripple in the gain of the optical amplifier. However, the noise figure of the optical amplifier improves. As a result, the overall performance of the optical amplifier is improved, particularly in conditions in which the attenuations produced by the variable optical attenuator are relatively large.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.